A double side polisher is a machine that polishes surfaces of both sides of a work piece at the same time. The work pieces are inserted in holes of a carrier respectively and the carrier with the work pieces is placed between upper and lower polishing plates on which polishing pads are plastered. Then, a planetary motion is provided to the carrier and a rotary motion is provided to the upper and lower polishing plates, while supplying slurry in the gap of the polishing plates and applying a predetermined polishing pressure to the work pieces by the polishing plates.
Although the amount of polishing of the work piece is usually monitored by means of polishing duration time, there is a case where it is necessary to detect the amount of polishing or material removal. Therefore, there are made some attempts to provide a thickness control device in a polishing apparatus.
Conventionally, there is known a thickness control device using a probe as disclosed in Japanese Examined Patent Publication No. S64-4126. The thickness control device disclosed in the document has a construction wherein a stylus of the probe is directed upward and the upper end of the stylus is in contact with the measurement chip fixed to the upper polishing plate. As the upper polishing plate goes down, with the advance of polishing, the chip of the upper polishing plate pushes down the stylus of the probe and the displacement, namely the amount of polishing, is measured by the probe.
Also, there is known another thickness control device using an eddy current distance sensor as disclosed in Japanese Examined Patent Publication No. S63-9943. This device mounted on the upper polishing plate measures the change of distance from itself to the lower polishing plate by detecting the change of impedance of the sensor.
Furthermore, in the thickness control device disclosed in Japanese Unexamined Patent Publication No. H10-202514, a reference aluminum plate is provided on the carrier, and distance L1 to the upper surface of the reference plate and distance L2 to the upper surface of an aluminum disk, or a work piece, are measured by the eddy current sensor and a difference between distances L1 and L2 is calculated to determine the thickness of the aluminum disk (work piece).